The invention relates to a projection exposure system for microlithography. This type of projection exposure system normally has a mask holding device in the form of a mask table or a so-called “reticle stage” for holding a mask or a so-called “reticle” with mask structures disposed on the latter. Furthermore, this type of projection exposure system normally comprises a substrate holding device in the form of a so-called “wafer stage” for holding a substrate in the form of a wafer and projection optics for imaging the mask structures onto the substrate.
With conventional projection exposure systems the quality of the image often suffers due to smearing. If the image position drifts over the wafer during exposure of a field, the latent image becomes smeared in the photoresist. This has the effect of superimposition errors or so-called “overlay” errors in the printed structures. These smearing problems occur to a particularly large extent with EUV projection exposure systems. EUV projection exposure systems expose structures with light with a wavelength in the extreme ultraviolet wavelength range, e.g. with a wavelength of 13.5 nm. In the roadmap of the semiconductor industry optical lithography plays an essential role in EUV. Only mirrors are considered here as optical components. With mirror optics a change in the position of the mirror and/or the mirror tilt setting leads as a first approximation to movement of the image. The requirements for mechanical stability of the optical components are substantially more stringent in comparison with refractive systems.
With conventional projection exposure systems the field position during the exposure of a wafer is monitored a number of times with appropriate adjustment or so-called “alignment” sensors, and corresponding correction measures are introduced. For this purpose the actual exposure process of the photoresist is interrupted. Between the control measurements one trusts in the short-term stability of the projection system. Conventional systems have relatively high short-term stability in comparison with EUV systems. The stability requirements for the image position when updating the conventional concept of image position control leads to an increase in the mechanical stability requirement in the image position of 1 nm with conventional systems to 0.2 nm with EUV systems over a period of 5 minutes. A main contribution to errors in the stability of the mirror positions is the thermal expansion of the mechanical base structure of the objective. In order to fulfil the stringent requirements set regarding image stability, one is currently following the path of using materials with extremely low thermal expansion coefficients for the structure of the projection objective. However, this type of material is extremely cost-intensive, sensitive and difficult to process.